The present invention relates to radio communication systems and, more particularly, to a multiple access method adopted for a radio communication system having a plurality of radio terminals.
For efficient utilization of limited communication medium, it is an indispensable technique to permit common use of a single transmission channel or a particular frequency band commonly by a plurality of terminals. In this technique, however, a problem of collision due to simultaneous accessing by a plurality of terminals is inevitable. In the Ethernet field, a multiple access system based on CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is usually adopted.
However, it is difficult to adopt the carrier sense system directly for a radio packet communication system based on TDMA/TDD (Time Division Multiple Access with Time Division Duplex). This is so because it is difficult for radio terminals in the TDMA/TDD system to monitor up-going carriers although down-going packets can be monitored. Another problem encountered is that it is difficult for the base station to discriminate whether a reception error generated therein to be due to a collision or to an interference.
For alleviating the above problems, ICMA-PE (Idle-signal Casting Multiple Access with Partial Echo) has been proposed (Umeda and Onoue, xe2x80x9cRadio Control Mobile Communication Random Access System with Partial Echoxe2x80x9d, Shingaku Gihoh, RCS 91-30). A system which adopts the ICMA-PE system for realizing point-to-multipoint packet communication, permits reducing the possibility of collision in a case where a plurality of terminals continuously transmit signal packets. Such a system is thus useful at low traffic levels.
Another multiple access system has also been proposed, which can ensure high throughput even at high traffic levels by adaptively and switchingly using two different access systems, i.e., one which is subject to collision such as the ICMA-PE system and the other one which is free from collision, such as a poling system (Japanese Patent No. 2746183).
A collision control method in the ICMA-PE system will now be briefly described with reference to the sequence diagrams of FIGS. 8(A) and 8(B).
Referring to FIG. 8(A), a radio base station CS informs terminals P connected thereto on radio of whether transmission is allowed or not by sending out a transmission allow signal (hereinafter referred to as IDLE signal) or a transmission non-allow signal (hereinafter referred to BUSY signal). The BUSY signal contains immediately previously received signal transmitted from a radio terminal and a poling address permitting discrimination thereof. The radio terminal which transmitted the signal thus can check whether or not the signal was received.
In the example shown in FIG. 8(A), a radio terminal PSI confirming an IDLE signal from the base station CS, transmits a first (head) signal packet D1(0, 1) as a division of transmission signal at the next transmission timing. In D1(0, 1), D1 indicates that the signal belongs to the radio terminal PS1, and (0, 1) indicates that the signal is the 0-th signal packet before next signal which is present and will be transmitted exists. In the actual ICMA-PE system, however, no sequence number (representing the sequential order of signal packet) is provided. In the Figure, however, sequence numbers are shown to facilitate the understanding.
Receiving signal D1(0, 1), the radio base station CS transmits next down-going signal BUSY-D1 (0), that is, it transmits BUSY signal together with the signal transmitted from the radio terminal PS1. Receiving BUSY-D1(0), the radio terminal PS1 recognizes that its own signal transmitted at the immediately preceding transmission timing has been received, and transmits the next signal packet. When a radio terminal PS2 monitoring the down-going signals detects a CRC (Cyclic Redundancy Check) error or the like, it transmits an NAK signal at the next transmission timing. FIG. 8(A) illustrates minimum control in the case when the radio terminal PS2 fails to continuously receive BUSY-D1(2) from the radio base station CS.
The NAK signal transmitted from the radio terminal PS2 at the transmission timing subsequent to the failure of correct reception of BUSY-D1 (2), collides with signal D1(3, 0) transmitted from the radio terminal PS1. When the radio base station CS detects this collision, it re-transmits BUSY-D1(2), i.e., the immediately previously transmitted down-going signal. When the radio terminal PS1 confirms that the next signal received from the radio base station CS is BUSY-D1(2), it re-transmits signal D1(3, 0), i.e., the immediately previously transmitted signal packet. It will be seen that in the ICMA-PE system, during continuous BUSY signal transmission no other radio terminal is allowed to transmit new signal. Thus, the frequency of collisions due to the multiple access can be greatly reduced.
In the ICMA-PE system, however, it is impossible to avoid collision of first signal packets. This will now be described with reference to the sequence diagram of FIG. 8(B).
When the radio terminals PS1 and PS2 simultaneously receive an IDLE signal from the radio base station CS and transmit respective first packets of transmission signal, a collision takes place. The radio base station CS recognizes the occurrence of a collision or an interference due to an CRC error or like error. However, since the idle state prevails at this time, the base station CS also transmits an IDLE signal at the next down-going signal transmission timing. Receiving this IDLE signal, the radio terminals PS1 and PS2 detect that a collision has taken place irrespective of first signal packet transmission, and set delay times for delaying the first signal packet transmission by generating random numbers. In FIG. 8(B), it is assumed that the radio terminals PS1 and PS2 set delay times corresponding to two and three transmission intervals, respectively.
If no transmission signal is transmitted from any other radio terminal until the lapse of the two transmission intervals, the radio terminal PS1 starts transmission. Subsequently, when receiving an IDLE signal from the radio base station CS after the end of the transmission from the radio terminal PS2, this time the ratio terminal PS2 starts transmission. However, if a radio terminal PS3 starts transmission while the radio terminals PS1 and PS2 are waiting during the delay times thereof (indicated as xe2x80x9ctransmission startxe2x80x9d in the Figure), the radio terminals PS1 and PS2 start re-transmission of signal packets when receiving an IDLE signal transmitted from the radio base station CS after the end of the signal packet transmission from the radio terminal PS3 (indicated as xe2x80x9csignal re-transmission startxe2x80x9d in the Figure).
However, since the radio terminal PS1 and PS2 start packet transmission simultaneously, a collision takes place once again, thus inevitably making it necessary for the terminals PS1 and PS2 to set further delay times. If the radio terminal S3 starts transmission once again during these delay times, transmission retention times elapse in the radio terminals PS1 and PS2. In the long run, it may result that the radio terminal PS1 and PS2 have to abandon the transmission.
As shown above, in the ICMA-PE system, with traffic increase it becomes difficult to avoid collision of first signal packets that are transmitted. The collision of first signal packets transmitted may particularly frequently occur when a high traffic level compared to the transmission bandwidth is brought about by the signal communication among a plurality of terminals. Although a terminal which transmitted collided signal can re-transmit the signal when receiving an IDLE signal from the radio base station after the lapse of a delay time that has been set by generating a random number in response to the collision occurrence, a terminal which is irrelevant to the collision may start transmission while the terminal relevant to the collision is waiting during the delay time thereof. In this case, the signal transmission from the terminal irrelevant to the collision is transmitting signal when the delay time is elapsed. Therefore, the terminal relevant to the collision has to wait until reception of an IDLE signal transmitted from the radio base station after the end of the transmission by the terminal irrelevant to the collision. The other terminal relevant to the collision is in like state, and thus the possibility of collision of the re-transmitted first signal packets is increased. It will be seen that although the communication performance of the system is fixed in itself, it is differently shared by the individual radio terminals. In addition, the communication performance of the overall system is degradated by frequent occurrence of collisions and resultant accumulation of delay times for delaying the transmission.
The present invention has an object of eliminating or alleviating the above two drawbacks, i.e., different sharing of the communication performance and degradation of the system communication performance, in high traffic level states.
According to an aspect of the present invention, there is provided a multiple access method in a radio communication system, which adopts a system for random access control by collision detection and permits multiple accessing of a first radio station by a plurality of second radio stations, wherein: the first radio station, when detecting a collision of reception signals, transmits an access-restricted transmission allow signal to the irrelevant-to-collision second radio stations for setting access restriction therein until reception of a normal transmission allow signal.
In the multiple access method, an irrelevant-to-collision second radio station, when receiving the access-restricted transmission allow signal before intended new signal transmission, suspends the execution of the new signal transmission until receiving a normal transmission allow signal from the first radio station.
In the multiple access method, a plurality of relevant-to-collision second radio stations, when receiving the access-restricted transmission allow signal after the lapse of a transmission delay time set at the time of collision occurrence, start the same transmission operation as when receiving a normal transmission allow signal.
According to another aspect of the present invention, there is provided a multiple access method in a radio communication system comprising a radio base station for detecting a collision of reception signals and a plurality of radio terminals, wherein: the radio base station, when detecting a collision of reception signals, transmits an access-restricted transmission allow signal to irrelevant-to-collision radio terminals for setting access restriction; each of a plurality of irrelevant-to-collision radio terminals, when receiving the access-restricted transmission allow signal at a reception timing subsequent to the timing of transmission signal transmission, sets a transmission delay time determined by a random number, and when receiving the access-restricted transmission allow signal after the lapse of the transmission delay time, starts the same transmission operation as when receiving normal transmission allow signal; and an irrelevant-to-collision radio terminal, when receiving the access-restricted transmission allow signal before starting new signal transmission, suspends the execution of the new signal transmission until receiving the normal transmission allow signal.
According to other aspect of the present invention, there is provided a multiple access method in a cordless telephone system comprising a provisional master terminal constituted by a slave terminal having a function of detecting a collision of reception signals and a plurality of slave terminals and capable of allowing inter-slave terminal communication, wherein: the provisional master terminal, when detecting a collision of reception signals, transmits an access-restricted transmission allow signal to irrelevant-to-collision slave terminals; each of a plurality of irrelevant-to-collision slave terminals, when receiving the access-restricted transmission allow signal at a reception timing subsequent to the timing of transmission signal transmission, sets a transmission delay time determined by a random number and, when receiving the access-restricted transmission allow signal after the lapse of the transmission delay time, starts the same transmission operation as when receiving a normal transmission allow signal; and an irrelevant-to-collision slave terminal, when receiving the access-restricted transmission allow signal before intended new signal transmission, suspends the new signal transmission until receiving the normal transmission allow signal.
Other objects and features will be clarified from the following description with reference to attached drawings.